High purity vacuum systems



p 1963 D. M. J. COMPTON ETAL 3, 04,049

HIGH PURITY VACUUM SYSTEMS F'iled Dec. 30, 1959 INVENTORS DINSDALE M. MPTON WALTER ER ;UORNEY United States Patent 3,104,049 HIGH PURITY VACUUM YSTEMS Dinsdale M. J. Compton, Rhinebeck, and Walter E.

Baker, Poughkeepsie, N.Y., assignors to International Business Machines Corporation, New York, N.Y., a

corporation of New York Filed Dec. 30, 1959, Ser. No. 863,061 6 Claims. (Cl. 230-69) This invention relates to the attainment of a high vacuum on the order of millimeters of mercury and in particular to the attainment of a high vacuum through the combined use of a pumping and a chemical reaction operation.

As the art of attaining a high vacuum has developed, it has been common practice to employ two types of pumping operations. A first pump reduces the pressure to the order of 10'" to 10 millimeters of mercury. This first pump is primarily conditioned for volume rather than extremely high vacuum values. At the pressure of 10" to 10- millimeters of mercury, a second type of pump is provided which is generally of the ion type which can then take the system down to the very high vacuums of 10- millimeters of mercury or lower. A problem has existed in the art, however, in that considerable difficulty is encountered in attempting to reach the pressure of 10- or 10 millimeters of mercury without introducing into the vacuum ingredients which tend to interfere later on with the uses in which the vacuum is to be employed. It has been found that the higher volume pumps capable of bringing the vacuum down to 10* or 10" millimeters of mercury tend to introduce impurities into the vacuum primarily in the form ort oil vapor and these impurities are later undesirable in applications to which the vacuum may ultimately be put.

What has been discovered is an improved technique for achieving a high vacuum without the use of pump fluids and hence the introduction of impurities from these pump fluids through the use of "a flushing operation with a gas accompanied by an absorbing of the highly reactive gas with a metal that will form a low vapor pressure hydride.

It is an object of the invention to provide an improved method of achieving a high vacuum.

It is an object of the invention to provide a clean vacuum at a value at which an ion pump can efliciently continue increasing the vacuum.

It is another object of the invention to provide a method of achieving a high vacuum by the steps of flushing, absorbing and then ion pumping.

It is another object of the invention to provide a high efiiciency getter for attaining a high vacuum.

Another object of the invention is to provide a vacuum by flushing out a system with a gas and reacting the gas in the system with uranium including the temperature of 1000" C. and subsequently ion pumping the system down to the desired vacuum.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as illustrated in the accompanying drawing.

In the drawing, the FIGURE is a schematic diagram of a high vacuum system illustrating the invention.

In accordance with the invention, a high vacuum is achieved by flushing out a vacuum system with a gas to remove all air and any other gas present and subsequently absorbing the gas from the closed off system with a metal that can form a low vapor pressure hydride. The technique of the invention, in addition to providing a high vacuum without the use of high volume fluid pumps which operate to contaminate the system, in addition, operates quite rapidly in that through the flushing and subsequent reaction operation the pressure of a gas for example hydrogen when uranium is used in the system is reduced from atmospheric to 2 10" millimeters in 5 minutes and to 9 10- millimeters in another 5 minutes. At such a pressure one of the many clean ioni zation pumps not employing fluids may readily take over to bring the system down to the desired 10 millimeters of mercury or lower.

An added advantage of the invention lies in the fact that the dangers of an explosion may be minimized with this type of system since the system may be flushed with nitrogen or argon before flushing with hydrogen. The metal uranium, when heated, has been found to react with all contaminating gases the vacuum (except the inert gases such as helium, Jargon, etc.) and the uranium may be re-activated to cause it to give up the materials it has extracted from the vacuum.

Referring now to the figure, a schematic diagram is provided of a system illustrating the invention. In the system of the figure, a reaction tube 1 is the object in which it is desired to obtain a clean contaminant free vacuum. The reaction tube 1 is positioned within a high temperature oven 2, shown dotted, which may be used to raise the temperature of the reaction tube and to bake out any impurities therein.

The reaction tube 1 is equipped with three openings 3, 4, and 5 respectively, each controlled by a suitable valve 6, 7, and 8 respectively. The opening 4 connects with a conduit 9 which passes through a cold trap 10, well-known in the art, to a closed container 11 wherein a quantity of a metal 12 is positioned covered by a diffusion type of plug 13, preferably of quartz wool or of sintered stainless steel. The metal 12 is a metal that can form a low vapor pressure hydride for example, thorium, zirconium, titanium, lithium and uranium. For purposes of illustration, the metal uranium will be described. A gas inlet 14 is made into the conduit 9 and controlled by a valve 15. An outlet passage 16- joins the conduit 9 and is controlled by a valve 17. The outlet passage 16 passes through a second cold trap 18. The outlet passage 16 travels in series through two pumps 19 and 20 to an exhaust 21. A control temperature oven for activation of the uranium 12 is shown dotted around the uranium container 11 as element 22. The element 19 is an oil difi usion pump capable of bringing the vacuum from 10- down to the desired value, and element 2% is a conventional high volume pump.

ably done by closing valves 7 and 15 and evacuating the container 11 through valve 17 with pumps 19 and 20. When a pressure of 10- to l millimeters of mercury is reached valve 17 is closed and furnace 22 heats the uranium to 225 C. At this temperature, valve 15 is opened and hydrogen admitted as long as it is taken up by the uranium 12. Valve 15 is then closed and the furnace 22 is heated to 400 C. At this temperature valve 17 is opened and container 11 is evacuated employing the pumps 19 and 20. This is repeated several times and reduces the uranium to a very fine active powder. The activation is completed by evacuating the container 11 to as low a pressure as can be reached, in the vicinity of millimeters. During this the temperature of the furnace 22 is maintained at 400 C.

With the activation complete, the valve 17 is closed. The container 1 is flushed out with nitrogen and then as many times with hydrogen as is necessary to remove all air and other gases by introducing the flushing gas in inlet 23 and exhausting it through outlet 24. This may be done during the activation time if desired. The flushing is done by closing valve 7 and opening valves 6 and 8. The container 1 can also be baked to drive impurities out during this operation by heating the oven 2 to the vicinity of 300 to 400 C.

The container 1 is then evacuated by closing valves 6 and 8 and opening valve '7 while the temperature is main taincd in the container 11 by holding the furnace 22 at 225 C. After approximately 2 to 10 minutes, the temperature of the furnace 22 may be lowered to 50 C. and then to room temperature. The hydrogen gas that was used to flush out the container 1 is absorbed by the uranium 12 and the pressure in the container 1 falls to typically 10 to 10 millimeters of mercury. During this stage, the container 1 may also again be baked out by heating with the furnace 2.

The system involving the container 1 is now ready to be further evacuated through the use of an ionization pump capable of taking the vacuum down to the desired value. The function of the cold traps 10 and 13 which are cooled with liquid nitrogen, is to collect stray impurities in the system. While the pumps 19 and 20 are illustrated as of the oil fore pump, oil diffusion pump type of system, it has been found that this technique involving flushing with gas does not introduce oil into the final clean system to be evacuated in any amount comparable with that introduced by the standard practice in the art of pumping directly with pump fluids. If desired, the metal 12 can be reactivated without any possibility of introducing pump oils by evacuating at the elevated temperature, with for example a water aspirator, not shown, connected to the reaction tube. The water aspirator is not shown, since it is not necessary to reach a very low pressure. By these means the metal 12 may be used over and over again.

It has been found, particularly, that the rate of absorption of hydrogen by uranium, goes to a maximum at a temperature of 200 C. to 250 C. The use of uranium has been found to be advantageous at higher temperatures since when the temperature of the furnace 22 is to be raised from 500 to 1000", the uranium will extract all impurities found in the container 1 since at this temperature, the uranium will react with practically every impurity encountered except hydrogen and inert gase. It has been found that the activated uranium metal at a temperature of 500 to 1000 will react with oxygen and nitrogen to give an oxide or nitride. It will also react to such gases as H 0 and hydrocarbons. These are decomposed to give solid reaction products such as uranium oxide and hydrogen. Further, on cooling to the 200 range the uranium takes up the hydrogen to form a uranium hydride and these impurities are thus removed from the system.

In order to aid in understanding and practicing the invention and in view of the fact that the technology is complicated to provide a starting point for one skilled in the art, the following set of specifications are provided, although it should be understood that no limitation is to be construed hereby for in the light of the above disclosure, many sets of specifications will become apparent to one skilled in the art.

Tube 1 Quartz 50 centimeters long, 10 centimeters in diameter.

Valves 6, 7, 8, 15 and 17 Alpert All Metal type, Granville Phillips Co., Pullman, Washington.

Cold traps 10, 18 Corning type No. 96600,

Corning Glass Co.

Container 11 Quartz 10 centimeters long, 2 centimeters in diameter.

Metal 12 Uranium-quantity 1 gm./

50 cc. volume to be evacuated.

Plug 13 Quartz wool.

Pump 19, oil diifusion type- Veeco Corp. No. EP2AB.

Pump 20, high volume fore pump type Welch Duo Seal Mechanical Pump No. 14028.

For the size tube of 1, a hydrogen flow of 500 cc. per minute is maintained in flushing and a nitrogen flow of cc. per minute is maintained in pre-flushing.

What has been described is a technique of solving the problem involving getting a vacuum from 10- down to 10* millimeters of mercury without the introduction of impurities through the use of a gas flushing operation and subsequently absorbing the gas with a chemical reaction employing a. metal that will form a low vapor pressure hydride as a getting agent.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A high vacuum system comprising, in combination a vacuum container; a first flushing system, including controllable inlet and outlet means to said vacuum container; and a second flushing system comprising a container of getter material of uranium, means for heating the getter to a predetermined temperature, an opening to said vacuum container, and pumping means, where said container of getter material, said opening to said vacuum container, and said pumping means are controllably connected together.

2. A high vacuum system comprising, in combination a vacuum container; means including controllable inlet and outlet means to said vacuum container for controllably flushing said vacuum container with a gas; and means comprising both a container of uranium getter controllably connected to said vacuum container and means including controllable gas flushing means, pumping means and heating means for activating said getter.

3. A high vacuum system comprising a vacuum container, a first valve, inlet means to said container connected to and controlled by said first valve, a second valve, outlet means to said container connected to and controlled by said second valve, a third valve, a conduit into said container controlled by said third valve, a getter material container, a quantity of uranium in said getter material container, means connecting said conduit to said getter material container, a fourth valve, gas inlet means to said conduit controlled by said fourth valve, 21 fifth valve, pumping means, tubing means connecting said pumping means, to said fifth valve, a first cold impurity trap surrounding said conduit between said getter material container and said third valve, a second cold impurity trap surrounding said tubing means between said pumping means and said fifth valve, first temperature control means surrounding said vacuum container and second temperature control means capable of reaching the temperature of -1 000 C. surrounding said getter material container.

4. The method of achieving a pump fluid impurity free vacuum of the order of 10" millimeters of mercury in a first container comprising the steps of: positioning a quantity of activated uranium Within a second container; flushing said first container with a gas capable of being adsorbed by said activated uranium; terminating said supply of flushing gas; connecting said first and second containers together such that said activated uranium getters said flushing gas to reduce the pressure within said first container to the range of 10* to 10- millimeters of mercury; and further reducing the pressure within said first container to about 10- millmeters of mercury by a vacuum pump that is free of pump-fluid impurities.

5. The method of achieving a pump fluid impurity free vacuum of the order of :10" millimeters of mercury Within a vacuum container comprising the steps of: flushing said container with a gas, sealing said container, connecting said container with a quantity of outgassed uranium to lower the pressure within said container to about 10"" millimeters of mercury, and thereazfiter pumping said container down to the order of 10- millimeters of mercury with a vacuum pump that is free of pumpfluid impurities.

6. The method of achieving a pump fluid impurity free vacuum of the order of 10' millimeters of mercury within a vacuum container comprising the steps of: flushing said container with a gas, sealing said container, connecting said container with a quantity of outgassed metal capable of absorbing said gas to lower the pressure within said container to about 10' millimeters of mercury, and thereafter pumping said container down to the order of 10* millimeters of mercury with a vacuum pump that is free of pump-fluid impurities.

References Cited in the file of this patent UNITED STATES PATENTS 

1. A HIGH VACUUM SYSTEM COMPRISING, IN COMBINATION A VACUUM CONTAINER; A FIRST FLUSHING SYSTEM, INCLUDING CONTROLLABLE INLET AND OUTLET MEANS TO SAID VACUUM CONTAINER; AND A SECOND FLUSHING SYSTEM COMPRISING A CONTAINER OF GETTER MATERIAL OF URANIUM, MEANS FOR HEATING THE GETTER TO A PREDETERMINED TEMPERATURE, AN OPENING TO SAID VACUUM CONTAINER, AND PUMPING MEANS, WHERE SAID CONTAINER OF GETTER MATERIAL, SAID OPENING TO SAID VACUUM CONTAINER, AND SAID PUMPING MEANS ARE CONTROLLABLY CONNECTED TOGETHER. 